1. Field of the Invention
The present invention relates to a power-assisted steering apparatus for wheeled vehicles, and more particularly to a power-assisted steering apparatus of the type which includes a control valve arranged to control hydraulic fluid under pressure supplied to a power cylinder therethrough from a hydraulic pump in response to the driver's steering effort applied thereto.
2. Description of the Prior Art
In such conventional power-assisted steering apparatuses as described above, various improvements have been proposed to eliminate detrioration in a return characteristic of the steering wheel. In Japanese Utility Model Early Publication No. 55-46051, for example, there has been proposed a power-assisted steering apparatus of the type which includes a return mechanism for mechanically effecting return movement of the steering wheel to a neutral position under the load of a spring assembled therein. In Japanese Patent Early Publication Nos. 61-115773 and 115774, there has been proposed a power-assisted steering apparatus of the type which includes a changeover valve arranged to apply hydraulic pressure to the power cylinder in a return direction when the steering wheel is returned to a neutral position.
In the former steering apparatus, however, a reaction force of the spring increases in accordance with an increase of the operation amount of the steering wheel, resulting in an increase of the driver's steering effort against the spring. In the latter steering apparatus, it is required to provide a sensor for detecting return movement of the steering wheel toward the neutral position and an actuator for effecting operation of the changeover valve in response to a signal from the sensor. This results in an increase of the manufacturing cost of the steering apparatus.